At present, strapdown inertial navigation is a type of booming and advanced navigation technology. Wherein, inertial elements including a gyroscope, an accelerometer and so on which are fixed to a carrier are directly used for measuring an acceleration of the carrier relative to an inertial reference system. Then the information on the speed, attitude angle and position in a navigation coordinate system can be achieved by an integral operation based on the Newton law of inertia, so as to guide the carrier from a start point to a destination. Besides, in the strapdown inertial navigation technology, mathematical operations comprising coordination transformation and differential equation solution on measured data by the gyroscope and the accelerometer are conducted by a control computer, in order to extract the attitude data and navigation data from elements in an attitude matrix to finish the navigation mission. In the strapdown inertial navigation system, a “mathematical platform” built based on updated data such as an updated strapdown matrix is in place of a traditional electromechanical navigation platform so as to achieve a simplified system structure, significantly reduced system volume and cost as well as inertial elements easy to install and maintain. Moreover, the strapdown inertial navigation system is independent of external system support, thus obtaining the information about attitude, speed and position on its own. It doesn't radiate any information to outside neither. Therefore, due to its advantages such as being real-time, independent, free of interruption, free from limitations of region, time and weather condition, as well as of comprehensive output parameters, it is widely applied in plurality of fields including aviation, sailing and traffic etc.
The strapdown inertial navigation system is usually composed of one inertial measurement system, one control computer, a control display and associated supporting components, wherein the inertial measurement system is the key component for such overall system. The inertial measurement system is equipped with a gyroscope and an accelerometer, and its operation principle is as follows: at first, triaxial angular speed of a carrier is detected by the gyroscope, and linear acceleration of a vehicle moving along the tri-axes is detected by the accelerometer; after that, in order to calculate some voyage attitude information such as instantaneous heading and inclination angle, the signal of angular speed detected by the gyroscope is subjected to an integral operation with respect to time by the control computer on one hand; on the other hand, the signal of acceleration detected by the accelerometer is subjected to an integral operation with respect to time so as to calculate the instantaneous velocity information; finally, a secondary integration is carried out to calculate the distance and position in the voyage during this period of time.
The inertial measurement system and its attitude solution technology are the key technical links that have an impact on properties of the strapdown inertial navigation system. This is because such inertial measurement and its attitude solution are the premises for controlling the track of the carrier. Thus, their precision and efficiency have a direct influence on the aging and precision of the navigation. Secondly, since the inertial measurement system has to bear vibration, impact and angular motion directly in a rigorous pneumatic environment, it is easy to bring about many destabilization effect and error effect, thereby becoming a weak link of the strapdown inertial navigation system. Thirdly, there are some challenges such as micromation and industrialization for the strapdown inertial navigation system. In particular, with the development of microelectronic technology, it is required to employ micro electromechanical inertial components with intermediate precision or even low precision for the purpose of producing such strapdown inertial navigation product with low cost and in batches.
When the carrier tends to miniaturization and micromation, since its foundation mass is much smaller than that of a conventional carrier, it will subject to more excitation and random vibration in the voyage dynamic environment and become more instable compared with the conventional carrier. Accordingly, in order to overcome the drawbacks of instable navigation, reduced precision and even shortened service life of electronic components, some targeted technical measures which are mainly in the aspects of mechanical structure, damping design and micro technology have to be proposed for the inertial measurement system.
FIG. 1 is a structure diagram for an inertial measurement system employed in a strapdown inertial navigation system of a small-sized UAV in the prior art. Wherein, a sensing support 11 is fastened to the interior of a housing 12 through a fastening screw, a damping unit 13 is formed by four rubber blankets, and the housing is fixed to a vehicle at its bottom. The sensing support is composed of three pieces of gyro circuit board 111, 112 and 113 perpendicular to each other (referring to FIG. 2), on which are arranged three one-axis gyroscope 111a, 112a and 113a, respectively. The gyro circuit board 111 in the horizontal position is a combined one. It is further provided with a triaxial accelerometer 111b besides the gyro 111a. These three gyroscopes should be installed on three orthogonal planes with their sensing axes perpendicular to each other to form an orthogonal coordinate system for measurement. On the combined gyro circuit board 111, the measuring axis of the tri-axial accelerometer 111b is in parallel with that of the gyro 111a. The combined gyro circuit board 111 is in direct connection with a conditioning circuit board 114 and a master processor circuit board 115 through a connector.
FIG. 3 illustrates an equivalent analysis for the damping structure of the above mentioned inertial measurement system. In the figure, a mass block M represents the inertial measurement system and its centre of mass is the m; the damping unit is indicated by {Ki, ci}, wherein the Ki stands for rigidity, the Ci stands for damping coefficient, and the subscript i stands for the number of the damping unit contained in a damper; since four rubber blankets are used as the damping units in the FIG. 1, i is equal to 1, 2, 3 and 4; B indicates a voyage carrier and P is the elastic centre of the damper. When the carrier B is during voyage, foundation excitation is produced for the inertial measurement system M. At this moment, in order to reduce the impact of the vibrations of the carrier B onto the inertial measurement system M, the damping unit {Ki, ci} absorbs and consumes the forced vibration energy from the carrier B, and it starts an elastic movement up and down while taking the point P as a centre.
There are some problems for the above mentioned inertial measurement system:
(1) the sensing support is composed of three circuit boards separated from each other, thus taking up too much space and resulting in significant differences among the rigidity on the three axial directions;
(2) since the damping units are installed outside of the inertial measurement system, they take up extra space; more importantly, when the inertial measurement unit is forced to vibrate, it is easy to have torsional vibration due to unbalanced rigidity and irrational mechanical structure;
(3) for the damper, its ideal sphere of action is limited to one-axis direction, that is, it can only attenuate the vibration from the vertical direction while having no effective suppression on the vibration from any other directions; as a result, linear vibration and angular vibration in different degrees of freedom can be coupled together and the damping band becomes narrow.